Vibrators of this type are usually attached to shaker tables on which products are mounted for vibration-tolerance testing.
Highly Accelerated Life Testing (HALT) and Highly Accelerated Stress Screening (HASS) applications utilize vibrators to identify product design and process defects by applying overstress conditions during design and manufacturing periods. These processes force failures in shorter times than would occur in the field. The identified design and process weaknesses can then be removed and the testing process can be repeated as necessary until there are no weaknesses that would cause early field failures.
Vibration-tolerance testing is done on many different types, shapes and sizes of products. These different products must be tested with vibrations having different magnitudes, directions and frequency spectra. The products are often tested by simultaneously applying to the product vibrations from multiple vibrators and along multiple axes or directions.
Over the years, many designs of shaker tables and actuators have been developed to test different products. Some have been tailored to one specific type or class of products and others have been designed to adapt to a range of products.
In all these cases, however, it is desirable to have the capability to adjust the characteristics of the vibrations to conform to the requirements of the product and its applications. Various methods have been devised to adjust and control the spectra of the vibrations applied to the products under test.
Most shaker tables use multiple vibrators of the same type and utilize the table characteristics and the mounting and orientation of the vibrators to obtain the desired vibration characteristics. Typically, these vibrators are of the pneumatic type and only permit the alteration of the magnitude and repetition frequency of their impulses by adjusting the air pressure supplied to them.
Some shaker table designs provide the flexibility to test various products with a single table and type of vibrator. For example, U.S. Pat. No. 5,969,256 to Hobbs teaches a modular shaker table having provisions for interchanging modules to alter the coupling of vibrations from the vibrators to the specimen under test. U.S. Pat. No. 6,502,464 to Hobbs and U.S. Pat. No. 4,154,161 to Nolan et al. teach the use of objects bouncing within a container to generate random force impulses from the periodic force impulses provided by a vibrator. U.S. Pat. No. 4,735,089 to Baker et al. teaches the use of a flexure member to provide damping of the vibrations.
Several types of vibrators have been devised for use in vibration testing. For example, U.S. Pat. No. 5,412,991 to Hobbs teaches a vibrator having a piston and sleeve with curvilinear surfaces which randomly vary the stroke length and reciprocation frequency of the piston. U.S. Pat. No. 5,493,944 to Felkins et al. teaches an actuator having a piston in a cylinder and a programmer which randomly bounces between the piston and the impact end of the cylinder thereby producing multiple random impacts during each cycle of the piston.
In the field of hydraulic hammers of the type used to break up concrete, street pavement and the like, U.S. Pat. No. 4,479,551 to Justus teaches an actuator for a hydraulic ram which utilizes a chamber containing oil to prevent metal fatigue due to metal-to-metal contact of impacting components.
However, in addition to an alterable shaker table and a vibrator which only permits adjustment of the frequency and magnitude of its force impulses, a vibrator in which the shape and frequency spectrum of its force impulses can be statically and dynamically controlled is also desirable.
It is therefore an object of this invention to provide a programmable vibrator which provides for static and dynamic adjustment of the temporal shape of a force impulse applied to a product either directly or indirectly through a shaker table. As is well known in the art, the shape of an impulse in the time domain is equivalent to a spectrum of frequencies in the frequency domain as described mathematically by the Fourier transform.